Interdiction method and apparatus with variable dwell time

ABSTRACT

A cable television interdiction apparatus comprises a microprocessor actuation and control means for actuating and controlling one or more frequency agile voltage controlled oscillators. The voltage controlled oscillators selectively jam only unauthorized premium programming transmitted from a headend to a particular subscriber. The method of interdiction comprises the steps of generating and storing voltage control words for operating the oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 07/476,041,filed Feb. 6, 1990 now abandoned, which is a continuation-in-part ofSer. No. 166,307 filed Mar. 10, 1988, now U.S. Pat. No. 4,912,760.Reference is also made to copending commonly owned applications, filedDec. 6, 1989, entitled "Picture Carrier Control Circuit For CableTelevision Interdiction or Jamming Apparatus", "Optimum Amplitude AndFrequency of Jamming Carrier In Interdiction Program Denial System","CATV Reverse Path Manifold System" and "CATV Subscriber DisconnectSwitch", U.S. Ser. Nos. 446,603; 446,602; 446,695; and 446,604,respectively.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to cable television systems and, moreparticularly, to a method and apparatus for applying remotely controlledand remotely applied interdiction or jamming signals to preventreception of unauthorized television channels.

2. Description of the Prior Art

At a headend of a cable television system, a scrambler is normallyprovided to encode premium television channels. The applied scramblingprecludes reception by an unauthorized converter/decoder at a connectedpremises. Data representing channels or tiers of programming areaddressably transmitted to a particular converter/decoder and stored inauthorization memory. As a result of the addressed transmission, asubsequently transmitted program is authorized in that the decoderportion of the converter/decoder will be selectively enabled to decodethe scrambled premium channel or program.

Several varieties of scrambling techniques are applied today. Eachmanufacturer has its own scheme which may be incompatible with others.Nevertheless, most popular scrambling systems today are based on syncsuppression, in which the sync information is hidden from the televisionreceiver's sync separator, usually by moving it to a level occupied bypicture information (moving the sync tip to an equivalent picture levelof 40 IRE units is common). Some systems modulate the picture carrierwith a sine wave phased to suppress the horizontal blanking interval.Most systems today switch to the suppressed level at the beginning ofthe blanking interval and switch out at the end. Most though not allsuppress the vertical blanking interval. Some systems dynamically invertthe video, either on a line-by-line or a field-by-field basis. This mustbe done carefully to avoid artifacts caused by inverting andre-inverting around different levels, and by differential gain and phaseof the system. Synchronization is restored either by the provision ofsynchronous amplitude modulated pulses on the sound carrier, by digitalinformation placed in the vertical interval or by phase modulation onthe picture carrier.

The provision of one scrambler per premium channel at the headend andthe inclusion of a descrambler in each converter/decoder at the premisesof the television receiver is particularly expensive. Furthermore, byproviding the converter/decoder on premises has turned out to be a greattemptation to service pirates who imaginatively seek ways to receivepremium channels. As a result, cable television equipment manufacturershave entered into a veritable war with such pirates resulting incomplicated service authorization protocols in some instances involvingmultiple layers of encryption by both in-band and out-of-band datatransmission further increasing the costs of the converter/decoder.

Furthermore, all scrambling systems leave artifacts in the horizontalblanking interval in the form of steps on the front and back porches.Normally these are not a problem, but if a television receiver does nothave adequate overscan, then the steps can show up as light bars on oneor both sides of the picture. Further, if a television receiver usesback porch sampling for automatic gain control and/or black levelrestoration, and the sampling period extends into the time of thedescrambling step, the television will show the wrong black level andmay show flicker in the picture. In systems in which pulse trains areapplied to the sound carrier, a buzz carried by harmonics of a 59.94 Hzsignal can be noticed in some television receivers.

Consequently, the cable industry has resorted to look for new technologyand to take a second look at technology developed in the early stages ofdevelopment of cable television such as the application of negative andpositive traps and more recent techniques such as interdiction.

Negative trap technology is viewed by many manufacturers as a viablealternative to sync suppression scrambling methods. A negative trap isbasically a narrow band reject filter. Traps are located at the drop toa subscriber's dwelling and attenuate a significant portion of a premiumtelevision channel rendering that channel unusable by the subscriber.

In the conventional embodiment, negative traps are made using L-C filtertechniques. The result is a notch with finite quality Q and finite shapefactor. In the case of a single channel negative trap, the center of thenotch is usually located at the picture carrier frequency of the channelto be removed. This technique, sometimes called a static negative trap,requires attenuation at the picture carrier of at least 60 dB to beeffective.

Negative trap systems have several advantages that make them attractivefor cable television applications. One primary advantage is the abilityto deliver a broadband cable television spectrum to the subscriber'sconverter/decoder. Conventional sync suppression systems utilizedescrambling set-top converter/decoders which deliver inherentlynarrowband signals. Negative traps are usually mounted outside thesubscriber's home (typically at the tap) and thereby minimize theexposure associated with placing hardware inside the subscriber'sdwelling. Finally, some cable television operators view the negativetrap as a more secure means of subscriber control than is syncsuppression, as picture reconstruction is viewed as substantially moredifficult.

However, the negative trap system requires hardware in locations whereno revenue is generated for the cable television system. Moreover,negative traps have several severe practical limitations. L-C bandreject filters have Q and shape factor limitations. Quality factors Qfor L-C filters are typically limited to less than 30. This means thatfor a negative trap located at channel 8 (picture carrier at 181.25 MHz)the 3 dB bandwidth of a negative trap is typically 6 MHz (or thebandwidth of a baseband television channel). This trap would result insignificant deterioration of the lower adjacent channel. Then thetelevision receiver tuned to the lower adjacent channel, rather thanhaving to contend with a 15 dB picture-to-sound ratio, may have tocontend with a sound carrier reduced an additional 6 dB or so. Frequencystability as a function of time and temperature is also a significantconcern. Many cable television system operators have instituted aregular negative trap change-out program based on the assumption thatafter a certain period of time and temperature cycling, frequency driftwill render negative traps useless.

Cascadability is another significant concern. Finite return loss andnon-zero insertion loss limit the number of single channel negativetraps which can be cascaded. As the number of services to be securedincreases, the negative trap decreases in appeal. Moreover a change inchannel line-up requires a significant investment in hardware andmanpower in this scenario.

Recently, a new type of negative trap has been introduced. The dynamicnegative trap consists of a notch filter that is designed to bemodulated with respect to frequency. The notch is centered about thepicture carrier but is deviated slightly from side to side. Thetelevision channel is rendered unusable by the introduction of unwantedamplitude and phase modulation on the picture carrier. This techniquerequires a notch depth significantly less than that of static negativetraps (typically 40 dB). Additionally, the intentionally introducedfrequency modulation reduces somewhat the requirement for frequencystability.

The dynamic negative trap, however, has several disadvantages. A powersource is required in order to accomplish the frequency modulation. Moresignificant is the parasitic modulation that this technique produces onthe adjacent television channels.

Positive trap systems also utilize a narrow band-rejector notch filter.However, unlike negative trap systems which are used to attenuate ortrap a premium channel transmission, the notch filter is used to restorethe premium television channel. In this scenario, an interfering signalis placed inside the premium television channel at the cable televisionheadend. This interfering signal is then removed at the subscribersdwelling by use of the notch filter. Ideally this notch filter removesonly the interference without removing a significant amount oftelevision information.

The positive trap technique is seen as having several advantages by thecable television system operator. It is considered advantageous to havethe interference present in the secured channels on the cable televisiondistribution plant (unlike the negative trap system in which thechannels to be secured are "in the clear" on the distribution plant). Itis very attractive from a financial standpoint to require subscriberhardware only at those locations where a subscriber wishes toreceive-secure service. Thus, any capital investment is associated witha point of revenue generation.

The conventional embodiment of the positive trap system utilizes L-Cnotch filters to remove the interfering signal. These L-C notch filterssuffer from the same imitations as do L-C negative traps discussedabove. Consequently, L-C based positive traps are limited to the lowerend of the cable television spectrum. Quality Q and shape factors havealso restricted the number of locations for the interfering signalwithin the television channel.

The location for the interfering signal in the conventional embodimentof the positive trap system is midway between the picture carrier andsound carrier. The energy density (and hence information density) inthis area of the spectrum is relatively low. One reason this locationwas chosen was that it minimized the impact of any televisioninformation removed along with the interfering signal by the notchfilter, and thereby improved the quality of the recovered televisionsignal. It would be expected that the jamming carrier would normallyhave minimal effect on the adjacent channel television picture unless atelevision has unusually poor rejection 2.25 MHz above the picturecarrier. The jammer does add another carrier which the tuner will haveto contend with, which might cause some degradation in a marginallyoverloaded ease.

Despite this location, the quality Q and shape factor limitations ofconventional L-C positive traps do remove a significant amount of usefultelevision information. The result is a noticeable "softening" of thetelevision picture as a result of attenuation of high frequencyinformation. Predistortion at the headend can improve this performancebut fails far short or being able to correct it completely. Thislocation for the interfering signal also facilitates the job of thevideo pirate. This pirate can easily tolerate a degraded signal andhence can recover a usable picture using techniques easily available(such as the classic twin lead quarter wave stub with an aluminum foilslider for fine tuning). Also, positive trap systems require a higherper premium channel cost than a negative trap system.

A relatively recent technique for premium channel control is theinterdiction system, so-called because of the introduction of aninterfering signal at the subscribers location. Most embodiments consistof a pole-mounted enclosure designed to serve four or more subscribers.This enclosure contains at least one microprocessor controlledoscillator and switch control electronics to secure several televisionchannels. Control is accomplished by injecting an interfering or jammingsignal into unauthorized channels from this pole-mounted enclosure.

For efficiency's sake, it is known to utilize one oscillator to jamseveral premium television channels. This technique not only reduces theamount of hardware required, but also maximizes the system flexibility.The oscillator output jamming signal frequency is sequentially movedfrom channel to channel. Consequently, the oscillator is frequency agileand hops from jamming one premium channel frequency to the next.

One such system is known from U.S. Pat. No. 4,450,481 in which a singlefrequency agile oscillator provides a hopping gain-controlled jammingsignal output to four high frequency electronic switches. Each switch isassociated with one subscriber drop. Under microprocessor control anddepending on which subscribers are authorized to receive transmittedpremium programming, the microprocessor selectively gates the jammingsignal output of the single oscillator via the switches into the path ofthe incoming broadband television signal to each subscriber.Consequently, an unauthorized subscriber, upon tuning to a premiumchannel, will receive the premium channel on which a jamming signal atapproximately the same frequency has been superimposed.

In the known system, it is indicated that sixteen channels may be jammedby a single voltage controlled frequency agile oscillator. With respectto one premium channel, this translates to a situation in which thejamming signal can only be present one sixteenth of the time or anapproximately 6% jamming interval. The rate of hopping is also indicatedat 100 bursts per second of jamming signal at a particular frequency ora 100 hertz hopping rate. Consequently, the effectiveness of the jammingsignal is questionable.

Cable television channels and, of course, premium service may extendover a wide range of frequencies, for example, from 100 to 350megahertz. In the known embodiment, the single oscillator provided mustbe frequency agile over a wide rage. It is further recognized that thejamming signal output of the single oscillator must be within a range of100-500 KHz above or below the video carrier frequency. Consequently, asynthesizer having an internal reference is provided to assure thereasonable accuracy of the jamming signal output of the oscillator tothe tolerable 100-500 KHz band above or below the video carrier.

It is indicated that the jamming signal is at a high relative power andis gain controlled to exceed the amplitude of the video carrier by 5 to20 dB. Because of the high output power relative to the premium channelvideo carrier power and the difficulty of precisely jamming the premiumchannel frequency, such an interdiction system leaves considerableopportunity for improvement. Because the oscillator is frequencyhopping, it spectrum tends to spread out around the picture carrier,generating a slightly different situation as far as the requiredadjacent channel rejection characteristics of the television areconcerned.

Firstly, it is important that the jamming frequency to be controlled soas to place the interference as close as possible to the picturecarrier. Secondly, it is also important to limit the peak amplitude ofthe interfering signal so as not to significantly exceed the video peakenvelope power in order to ensure that there are not residual artifactson adjacent channels. However, in the known system, adjacent channelartifacts are also created since the jamming signal is intentionallyplaced below the video carrier and consequently proximate to an adjacentchannel. Also, the rate of frequency hopping is limited in the knownembodiment as a result of its application of conventional frequencycontrol techniques during the hopping process.

The known interdiction system has proven to be particularly susceptibleto adjacent channel artifacts from the above described amplitude andfrequency and frequency selections which can dissatisfy subscribers.Furthermore, the subjective perception of the depth of jamming anunauthorized premium channel is relatively unsatisfactory resulting fromthe limited maximum six percent jamming interval when sixteen premiumchannels are jammed by a single oscillator and the relatively low rateof frequency hopping.

SUMMARY OF THE INVENTION

Many of the above-stated problems and related problems of the prior arthave been solved with the principles of the present invention, atelevision channel interdiction method and apparatus capable of remotelycontrolled jammed depth and frequency at reduced power. Afterconsiderable investigation into the known art and throughexperimentation, it has been determined that an optimum placement of ajamming signal is within the approximate range extending from the videocarrier to 250 kiloherz above the video carrier, a jamming signalplacement much below the video carrier creating artifacts in the nextlower adjacent channel. Such a placement is between the video carrierand the audio carrier for the same premium channel. Also, from the headend, the jamming carrier may be precisely established at a frequencyresolution of 50 KHz as a digitally step-wise selectable frequencywithin the 250 KHz range above the video carrier. A ten bit voltagecontrol word is applied by way of a digital to analog converter to avoltage controlled oscillator to control the frequency of the jammingsignal within this frequency range or to provide a jamming signaloutside the range, for example, if the audio carrier is to beintentionally jammed. Furthermore, to insure the accuracy of thefrequency of the jamming signal and to limit jamming signal frequencyharmonic interference, a plurality of oscillators are provided, eachoperating within a particular narrow band of the cable televisionspectrum. The sum of all such narrow bands shall be equivalent to theentire spectrum over which jamming is desired, recognizing that thecable television spectrum to be jammed may itself be discontinuous orthat some overlap in bands may be desired. In particular, four separateoscillators are provided whose outputs are separately filtered toeliminate the appearance of harmonies of the jamming signal output whichcan interfere with television reception on other channels higher in thespectrum. Each oscillator may be intentionally limited to jamming amaximum of four channels within its band resulting in approximately afactor of four improvement in jamming interval over the prior art.Furthermore, each plurality of oscillators is provided on a persubscriber or per drop basis.

Also, in accordance with the principles behind the present invention,the jamming signal power is limited within the range of -2.5 dB and +6.5dB with a +2 db nominal with respect to the video carrier power level.Consequently, there is less chance of adjacent channel interference thanin the known prior art system.

Furthermore, it has been determined that jamming depth, the subjectiveperception of one viewing a .scrambled television channel on a number ofdifferent television receivers, is improved by improving the frequencyhopping rate to approximately four kilohertz, a factor of twentyincrease in rate over the known system, all other parameters being equalsuch as amplitude and frequency of the jamming signal. As will bediscussed herein, the present embodiment is capable of achievingfrequency hopping rates of this magnitude because it is not limited byconventional frequency locking techniques.

The microprocessor of the present apparatus further controls theprovision of power to the plurality of oscillators. If the subscriber isauthorized to receive all premium channels within the band secured by agiven oscillator, that oscillator may be powered down for the duration.Furthermore, no residual jamming signal output power will pass throughan intentionally open switch during a powered up condition as mightoccur in the prior art interdiction system.

Common circuitry is shared by a plurality of subscribers, for example,up to four, and is housed in a pole-mounted, strand-mounted or pedestalhousing. The common circuitry comprises automatic gain control circuitryfor regulating the level of video carrier. The common circuitry alsocomprises a data receiver, a data decoder and a microprocessor which maybe individually addressed. The common circuitry separately decodes foreach addressed and in service subscriber module any commands and datatransmitted from the headend. The microprocessor of the common circuitrycommunicates with the microprocessor of the subscriber module anydecoded data related to the particular subscriber served by thatsubscriber module. The decoded data, for example, indicates individuallyaddressed channel or program authorization data or globally transmittedchannel frequency and jamming depth data received from the headend forstorage in microprocessor memory.

During a normal mode of operation the microprocessor of the subscribermodule actuates or powers up each required oscillator and transmitsfrequency data toward all required oscillators for jamming any and allunauthorized channels at a jam factor selected for each particularpremium channel. In particular, a sixty-four position memory may bereserved for storing ten bit voltage control words. An algorithm of thesubscriber module microprocessor loads the voltage control word memorydepending on the level of service chosen by the subscriber. In oneextreme where a particular subscriber is authorized to receive allpremium channels but one, three of the oscillators may be powered downand the remaining oscillator is capable of continuously jamming the oneunauthorized channel resulting in a 100% jamming interval.

If a particular subscriber at a given point in time has subscribed tonone of the sixteen channels offered, all four oscillators aresequentially triggered and sixty-four voltage control words are providedin a pseudo-random sequence toward the four oscillators. The applicationof such a pseudorandom sequence can thwart pirating.

Jam factor as defined herein is a parameter selectable and globallytransmitted by the headend to equate to the relative degree of jammingto be applied to different premium channels. For example, it may beappropriate to jam highly restricted programming at a higher jam factor.In accordance with the present invention, a total of sixteen voltagecontrol words may be allocated to one premium channel. Consequently, thesixteen control words are analogous to jamming time slot intervals whichcan be allocated by the headend to improve jamming depth. For example,if three premium programs are provided by the headend over channelswithin the allocated band of one oscillator at a particular point intime, these sixteen time slots or their responsive jam factors may beallocated at eight, four and four respectively (totalling sixteen) toeffectuate, for the least jammed channel and allowing a five percentoverlap; a minimum 20% jamming interval. As already indicated, if thesubscriber subscribes to all of these three channels, the microprocessoralgorithm will power down or deactuate the oscillator entirely orincrease the jamming interval proportionate to the degree of premiumservice subscribed to and the assigned jamming factors.

According to a novel aspect of the present invention, the amount of timea particular channel is jammed can be varied to enable an operator tomore heavily jam (increase the jam factor) of certain channels which mayrequire greater security. Moreover, the dwell time can be variedrandomly and/or more than one time slot per cycle can be associated witha particular channel. Also, a plurality of consecutive time slots may beallocated to a particular channel.

Periodically, and at power up, the present apparatus enters acalibration mode of operation, for example, at approximately thirtyminute intervals. From the front end, premium channel frequency data isglobally transmitted for storage in memory of the microprocessor of thecommon circuitry. The common circuitry microprocessor in turn calculatesa divide by factor for a programmable prescaler of the subscriber moduleand an expected time between frequency counts and forwards thesecalculations to the microprocessor of each subscriber module.

The programmable prescaler or frequency divider is provided in afeedback path from the plural oscillators to the microprocessors of thesubscriber module. During the calibration mode, only one oscillator ispowered at a time. The transmitted and stored frequency data istranslated into a best guess voltage control word. As a result, thejamming signal frequency of the only powered-up oscillator is fed backthrough the prescaler which divides down the high frequency output forcounting by the microprocessor. The microprocessor calculates a count inaccordance with the known time interval between received outputs of theprescaler. The count is then compared with the expected count and thevoltage control word adjusted accordingly. After a maximum of ten suchcalculations, starting with the most significant bit of the voltagecontrol word, a particular control word is precisely established involtage control word memory. In sequence, all sixteen control words ofeach oscillator or all sixty-four words are precisely established, theentire procedure requiring only a fraction of a second. Consequently, nomeaningful intelligence can be obtained during the calibration mode if asubscriber coincidentally attempts to view an unauthorized premiumchannel. Thus, the calibration mode in combination with the provision ofplural narrow band oscillators assures jamming signal frequency control.

As a consequence of the calibration mode of operation and the structureof the present apparatus, the jamming carrier can be practicallypositioned anywhere within the 250 kilohertz band above the videocarrier or even elsewhere if desired, for example, for jamming the audiocarrier. During the approximately thirty minute interval in times oftemperature variations, there is an opportunity for frequency drift of agiven jamming signal frequency. However, the drift, if existent, isactually desirable in the sense that such a drift will thwart any wouldbe pirates attempting to trap the jamming signal with a notch filter. Asalluded to before, the jamming signal frequencies may also beintentionally varied by varying the voltage control words for jamming agiven premium channel. Furthermore, they may be applied in apseudo-random sequence. Consequently, a would-be pirate would have tofollow the same pseudo-random sequence and sequentially actuate aplurality of notch filters, all at the same frequencies as arerepresented by the associated voltage control words as well asanticipate the natural frequency drift previously alluded to in order topirate the premium television signal.

Importantly also, the application of a calibration mode of operation, asdistinct from a normal mode of operation, permits the present apparatusto achieve much higher frequency hopping rates than the known systemduring the normal mode of operation. Because there is no requirement forthe application of slow conventional frequency locking techniques duringthe normal mode of operation, a desirable three to four kilohertzhopping rate is achievable.

These and other advantages of the present method and apparatus forproviding remotely and addressably controlled interdiction will now beexplained with reference to the drawings and the following detaileddescription of one embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall system block diagram showing the inherentcompatibility of the present interdiction apparatus with existent cabletelevision systems comprising premium channel scramblers, addressabledata transmitters, and subscriber converter/decoders.

FIG. 2 is a block schematic diagram of an addressable common controlcircuit for the plurality of provided subscriber modules in accordancewith the present invention comprising a broadband a signal tap, amicroprocessor, a data receiver and decoder and an automatic gaincontrol circuit.

FIG. 3 is a block schematic diagram of one subscriber module comprisinga microprocessor for selectively actuating and controlling the outputfrequency provided by each of four voltage controlled oscillators suchthat during a normal mode of operation sixteen premium channels may bejammed at a minimum twenty percent jamming interval and, during acalibration mode, a feedback path is provided to the microprocessorthrough a programmable prescaler to precisely establish jamming signalfrequencies.

FIG. 4 is a frequency plan for allocating the broadband cable televisionspectrum among four separate bands, each of which bands comprising aplurality of channels greater than or equal to four but, of whichplurality, only four channels may be jammed at a 20% jamming interval.

FIG. 5 is a detailed block schematic diagram of one embodiment of afeedback loop structure for implementing the calibration mode ofoperation of the present invention.

FIG. 6 is a block diagram of the voltage control word memory inconnection with the sequential provision of oscillator jamming frequencysignal outputs during a normal mode or operation.

FIG. 7 is a timing diagram for the embodiment of FIG. 3 during a normalmode of operation in which each interdiction control signal isparticularly depicted.

FIGS. 8(a-d) depicts several exemplary jamming patterns for jammingplural channels with different dwell times.

DETAILED DESCRIPTION

Referring more particularly to FIG. 1, there is shown a general blockdiagram of a cable television system employing the principles of thepresent invention. By cable television system is intended all systemsinvolving the transmission of television signals over cable towardremote locations. For example, a cable television system may comprise acommunity antenna television distribution system, a satellite signaldistribution system, a broadcast television system, a private cabledistribution network, either industrial or educational, or other formsof such systems. Each remote location of a television receiver maycomprise the location of a particular subscriber to a subscriptiontelevision service, plural subscribers, single subscribers having pluraltelevision receivers or private locations in a private cabledistribution network. Consequently, the term subscriber, when used inthis application and the claims, refers to either a private subscriberor a commercial user of the cable television system. Headend 100 as usedin the present application and claims is defined as the connecting pointto a serving cable 110 for distributing television channels tosubscriber locations. For reference purposes, an Electronic IndustriesAssociation (E.I.A.) standard cable television frequency allocationscheme is employed and referred to herein; however, by means of thefollowing disclosure of the present invention, one may apply theprinciples to other known standards or non-standard frequencyallocations. Furthermore, a National Television Subcommittee (N.T.S.C.)standard composite television signal at baseband is generally consideredin the following description; however, the principles of the presentinvention apply equally to other standard and non-standard basebandtelevision signal formats.

Headend 100 comprises a source of television programming 101. Televisionprogram source 101 may be a satellite television receiver output, aprogram produced by a television studio, program material received overa microwave or broadcast television link, a cable television linkoutput, or any other source of television programming consistent withthe present invention. The program source material need not be limitedto conventional television but may comprise teletext, videotex, programaudio, utility data, or other forms of communication to be delivered toa remote location over the serving cable 110.

Program material provided by source 101 may be premium or otherwiserestricted or desirably secured from receipt at unauthorized receiverlocations. To this end, each channel or program to be secured isgenerally scrambled by scrambler 102 provided at headend 100. By the useof the term premium channel or premium programming in the presentapplication and claims is intended a channel or program which is desiredto be secured from unauthorized receipt either because of its premium orrestricted status.

Normally, all premium programming in known cable television systems isscrambled. However, in accordance with the present invention, premiumprogramming is transmitted in the clear, and interdiction is applied atinterdiction apparatus 130 to jam reception of unauthorized premiumprogramming.

Consequently, during a transition period in which headend 100 providesscrambled television programming as well as premium programming in theclear, a scrambler 102 will be provided so long as converted/decoders150 are provided to subscribers for unscrambling scrambled/programtransmission. In certain instances, converter/decoders 150 may beentirely replaced by interdiction apparatus 130 of the presentinvention.

Also, at the headend, there is normally an addressable data transmitter103 for transmitting global commands and data to all subscribers oraddressed communications for reception by a unique subscriber. Such datatransmission may be conducted over a separate data carrier from thecable television spectrum, for example, at 108.2 megahertz. It may alsobe transmitted over an unused default channel from the televisionspectrum. Global commands generally take the form of operation code anddata while addressed communications further comprise the unique addressof a particular subscriber.

In another alternative embodiment, such communications may take the formof in-band signals sent with a television channel superimposed upon anaudio carrier during, for example, the vertical interval of the videosignal. Such data communications further complicate data reception atintervention apparatus 130 in accordance with the present invention andare desirably eliminated. However, in band signalling is sometimesrequired for the operation of certain converter/decoders 150 known inthe art.

Consequently, headend 100, cable television distribution cable 110, andconverter/decoders 150 and television receivers 170 at a typicalsubscriber premises 181 comprise a typical known cable televisionsystem. Channel program or authorization data is transmitted via anaddressable data transmitter 103 over a cable 110. At a pole 120 or froma pedestal 140 at underground cable locations, the serving signal isdropped via drop 115 to a subscriber location. Drop 115 is connected toa conventional converter/decoder 150 which serves several functions.Responsive to an addressed communication from headend transmitter 103,channel or program authorization data is updated in an authorizationmemory if the address associated with the addressed communicationmatches a unique address of the subscriber decoder 150. For example, thesubscriber address may comprise a plurality of bits over and above theactual number of subscribers in a system, additional bits insuring thesecurity of the address. The premium channel or program is then storedin the authorization memory of the converter/decoder 150. Televisionprogramming is normally converted to an otherwise unused channel such aschannel 3 or 4 of the television spectrum by a converter portion ofconverter/decoder 150. Its premium status is checked against the datastored in authorization memory. If the programming is authorized, thedecoder portion of the converter/decoder is enabled to decode authorizedscrambled premium programming.

The provided television receiver may be a conventional televisionreceiver 170 or may be a so-called cable ready television receiver 171.Because of the advent of cable ready television receivers 171, there isno longer a requirement at a subscriber premises 181 for the converterportion of the converter/decoder 150 as a converter is built into suchtelevision receivers.

In accordance with a cable television system provided with interdictionapparatus 130 of the present invention, a housing is mounted on a strandsupporting cable 110, to a pole 120, or provided via a pedestal 140.Inside the housing is common control circuitry for tapping into thebroadband television and data transmission spectrum. Referring to thefirst pole 120 from the left of FIG. 1, there is shown a strand-mountedapparatus serving two drops 115 may be served by interdiction apparatus130. Besides the common control circuitry, four plug-in subscribermodules may be provided for one housing. Also, if desired, additionalservices may be provided via other plug-in units of the housing such asimpulse pay-per-view, subscriber polling involving two way datacommunication, meter reading, energy management or other services.

Desirably, all equipment 161 may be removed from the subscriber premises182. However, for the provision of additional services, some on-premisesequipment may be unavoidable. For purposes of this description, premises182 will be assumed to include at least one non-cable ready conventionaltelevision receiver 170. Consequently, subscriber equipment 161 must atleast comprise a tunable converter for converting a received cabletelevision channel to an unused channel such as channel 3 or 4 forreception on conventional television receiver 170.

Power for interdiction apparatus 130 may be provided over the cable fromthe headend 100 or be provided via the subscriber drop 115 or by acombination of such means. Foreseeably, power may be even provided byrechargeable means such as solar cells or other external or replaceableinternal sources such as batteries. Consequently, subscriber equipment161 may also comprise a source of power for interdiction apparatus 130.

Interdiction apparatus 130 may be secured in a tamper-resistant housingor otherwise secured such as a locked equipment closet of an apartmentcomplex. If located in a place exposed to the elements, the housingshould be water-tight. Also, the housing should be designed to precluderadio frequency leakage.

At premises 183, the subscriber is presumed to have a cable-readytelevision receiver 171. Consequently, subscriber unit 162 may beentirely eliminated or comprise simply a power feed to interdictionapparatus 130.

Premises 184 pictorially represents a subscriber location served by anunderground cable 110 via a plurality of pedestals 140, in which cabledistribution amplification and branching equipment and drops 115 arenormally provided. In accordance with the present invention, pedestal140 may comprise an off-premises housing for interdiction apparatus 130.Subscriber equipment 162 may comprise a converter, an additional servicedevice and a power unit as described in reference to subscriberequipment 161 or nothing at all as described in reference to subscriberequipment 162.

Interdiction apparatus 130 is uniquely addressable by headend 100 justas is converter/decoder 150. If two bits of a plural bit uniquesubscriber address are associated with uniquely identifying one plug-inslot for one of four subscriber modules, common control circuitry may beuniquely addressed with remaining address data not used to secure thedata communication. Just as premium programming is transmitted in theclear and since no data communication is necessarily required withsubscriber premises, a subscriber address need not be transmitted in asecure form in accordance with the principles of the present invention.Nevertheless, address security may be desirable so long asconverter/decoders 150 or other unique address requisite equipment isprovided at a premises.

Interdiction apparatus 130 comprises addressable common controlcircuitry and up to four plug-in subscriber modules. Upon receipt ofsubscriber specific premium program or channel authorization data, thedata is stored at interdiction apparatus 130. Interdiction apparatus 130further comprises automatic gain control circuitry of the common controlcircuitry. Channel interdiction circuitry associated with eachsubscriber module jams unauthorized premium programming dropped via aparticular drop 115 to a particular subscriber. Consequently,interdiction apparatus 130 is reasonably compatible with addressableauthorization data transmission known in the art. No scrambling ofpremium channels (and no resulting artifacts) is necessary or desirable.Furthermore, no additional forms of service security are necessary suchas channel encryption, in-band channel or tier verification or othersecurity measures. The would-be service pirate must attempt to remove aparticular pseudo-randomly timed jamming signal placed at a varyingfrequency or seek to tamper with the off-premises apparatus 130 orderive a signal from shielded and bonded cable 110 which should likewisebe maintained secure from radio frequency leakage.

The common control circuitry of interdiction apparatus 130 will not bedescribed by means of the block diagram FIG. 2 for serving foursubscriber modules in accordance with the block diagram FIG. 3.Referring particularly to FIG. 2, a feeder cable 110 is shown enteringinterdiction apparatus 130 at FEEDER IN and leaving at FEEDER OUT. PowerPWR may be provided via the feeder cable by means of a subscriber dropor locally by internal or external means. Depending on the source ofpower PWR, input power may be of alternating or direct current.

A directional coupler 210 which may be in the form of a plug-in moduletaps into the broadband serving cable 110. A broadband of radiofrequency signals is thus output to highpass filter 220. Highpass filter220 passes a band of frequencies comprising at least the data carrierfrequency or frequencies (in a bi-directional application) and the cabletelevision channel spectrum. Referring briefly to FIG. 4, the cabletelevision spectrum may comprise a frequency band from at least 120 MHzto 350 MHz.

An automatic gain control circuit comprises variable attenuator 230, RFamplifier 233, directional coupler 232, and AGC control circuit 231. Theautomatic gain control circuit appropriately regulates the broadband RFsignal power to fall within established limits.

Also connected to directional coupler 232 is a data receiver 240 forreceiving data from the addressable data transmitter 103 located atheadend 100. Data receiver 240 receives data transmitted, for example,over a data carrier of 108.2 megahertz and provides unprocessed data todata decoder 250. In accordance with an established protocol, such datamay be in the form of an operation code, a subscriber unique address andassociated data. Data decoder 250 processes the data and provides theseparately transmitted data to microprocessor 260 for furtherinterpretation in accordance with a built-in algorithm. Microprocessor260 is most efficiently chosen to alleviate as many responsibilitiesfrom any microprocessor provided for an individual subscriber module andso is most conveniently an eight bit microprocessor having eightkilobytes of internal code such as a Motorola 68HCO5C8.

Received data may be stored in uninterruptable memory 270 bymicroprocessor 260. Data may be temporarily stored in memory 270 or morepermanently stored and subsequently downloaded when needed to asubscriber module via a serial peripheral interface bus connectingmicroprocessor 260 with separate microprocessors associated with eachprovided subscriber module.

Microprocessor 260 consequently interprets both global communicationsaddressed to common control circuitry or communications addressed tounique subscriber modules. If appropriate, microprocessor 260 ignoresglobal or addressed communications to other interdiction apparatus 130or to converter/decoders 150 (FIG. 1). Examples of global communicationspeculiar to interdiction apparatus 130 are premium channel frequencydata and jamming factor data for each premium channel or channels overwhich premium programming at a particular point in time is provided viaheadend 100. Examples of addressed communications include communicationscomprising premium channel or programming authorization information orcommunications instructing the common control circuitry to deny orprovide service to a particular subscriber.

If two way services over the serving cable are anticipated, a datatranscriber (not shown) must be provided in the common control circuitryof FIG. 2 or a separate telephone link from the subscriber location tothe headend may be provided. Serial peripheral interface bus 290 may bea two way communications link by way of which link microprocessors 300(FIG. 3) associated with subscriber modules may, at least, providedstatus reports to microprocessor 260 upon inquiry.

Radio frequency splitter 280 provides broadband radio frequency signalscomprising at least the cable television series spectrum of FIG. 4separately to each subscriber module that is provided.

If a reverse path is required for special additional services, a signalcombiner (not shown) or a plug-in special service module may be providedfor receiving communications from each of the four subscriber modules inan opposite manner to splitter 280. Certain data may be transmitted backtoward the headend via the special service plug-in module (also, notshown) associated with the additional special service.

Referring more particularly to FIG. 3, there is shown an overall blockschematic diagram of a subscriber module in accordance with the presentinvention. A microprocessor 300 is associated with a particularsubscriber module and communicates with microprocessor 260 of FIG. 2over a serial peripheral interface bus. Microprocessor 300 may comprisean eight bit microprocessor equipped with only two kilobytes ofmicrocode, this microprocessor being relieved of overall controlresponsibilities by microprocessor 300. Consequently, microprocessor 300may conveniently comprise a Motorola 68HCO5C3 microprocessor or similarunit.

A reverse path may be provided via a lowpass filter 392 to a specialservice module (not shown in FIG. 2) of common control circuitry asdescribed in FIG. 2 from a corresponding special service module on thesubscriber premises. Such as reverse path is completed to the subscribervia terminal OS. Also power may be transmitted up the subscriber drop tothe module of FIG. 3 and withdrawn at terminal OS.

The broadband radio frequency television spectrum signal from FIG. 2 isprovided to terminal IS. Referring to the path connecting terminal IS toterminal OS, there are connected in series a service denying switch 389,an amplifier 387, a jamming signal combiner 384, and a high pass filter391. Service denying switch 389 is under control of microprocessor 300.In the event of an addressed communication from headend 100 indicating,for example, that a subscriber is to be denied service for non-paymentof a bill, service denying switch 389 may be opened. In addition, a highfrequency amplifier 387 may be powered down under control ofmicroprocessor 387 whenever service is to be denied. Otherwise,amplifier 387 may be set at discrete gain levels, under microprocessorcontrol, to provide supplemental gain to the broadband television signalif a subscriber has a plurality of television receivers over and above anominal amount.

Jamming signals are interdicted at directional combiner 385 undermicroprocessor control. Because of the directional characteristic ofamplifier 387, jamming signals cannot inadvertently reach the commoncontrol circuitry of FIG. 2 or the serving cable 110. Jamming signalsare interdicted at a level approximately within a range of -2.5 dB to+6.5 dB or +2 dB nominal of the video carrier power level of theunauthorized premium channel frequency to be jammed. They are mostconveniently interdicted for video carrier jamming approximately withina range of frequencies extending from the video carrier to +250Kilohertz above the video carrier toward the audio carrier of thechannel to be jammed. In accordance with the present interdictionapparatus, the frequency is selectable by the headend 100 and so may bechosen to jam the audio carrier at a frequency closer to that carrier ifdesired. Also, the power level of the jamming signal may be varied viaglobal data transmissions, if, for example, audio carrier jamming isdesired. Such interdiction on a per channel basis between the video andaudio carriers minimizes adjacent channel artifacts.

Highpass filter 391 prevents any return path signals from reachingcombiner 385 and passes the broadband spectrum including any jammingsignals toward terminal OS. Reverse path signals, for example in thisembodiment, if present, may be radio frequency signals below 100megahertz. The broadband television spectrum is presumed to be in the100-350 megahertz range consistent with FIG. 4. However, interdiction ofpremium channel viewing may be allocated anywhere desired within abroader or discontinuous cable television spectrum to be jammed.Consequently, filters 391 and 392 are designed in accordance with thisor similarly selected design criteria to block or pass broadbandtelevision or reverse path signals as required.

Microprocessor 300 controls one or more voltage controlled oscillators.According to a preferred embodiment, four such oscillators (e.g.,341-344), each of which jams premium channel frequencies within anallocated continuous range of frequencies, may be used. However, theteachings of this invention are applicable to any number of oscillators.Since premium programming may be transmitted anywhere in the cabletelevision spectrum, the sum of all such allocated portions comprisesthe entire television spectrum to be jammed. In accordance with thepresent inventions, the television spectrum to be jammed may comprisediscontinuous portions or intentionally overlapping portions.

Referring briefly to FIG. 4, the spectrum allocation to the plurality offour voltage controlled oscillators in one embodiment will be discussedin view of certain principles. Firstly, it is desirable to eliminatejamming signal harmonic interference to authorized channels within theallocated band. For example, a harmonic of a relatively low frequencysignal, for example, 100 MHz can interfere with a channel at a harmonicof this frequency in the upper part of the cable television spectrum. Inother words, the allocated band should be limited for an oscillator tofall within one third of an octave, and, consequently, all frequencyharmonics may be blocked by filters 351, consequently all frequencyharmonics may be blocked by filters 351, 352, 353 and 354 associatedwith each oscillator. Oscillator 341 denoted OSC 1, for example, isactive in a band extending from 126 to 158 megahertz while filter 351block harmonics: above the included channels 15-20 of the midband.

Cable headend service providers tend to select premium channelallocations in the midband range covering channels 15-22. Consequently,the band of oscillator 342, for example, may be selected to overlap theband allocated to oscillator 341.

In order to achieve a jamming interval of 20%, each oscillator may berestricted to jamming only four premium channels. As will be describedin connection with a discussion of FIGS. 5, 6 and 7, jamming depth maybe automatically increased for a particular subscriber dependent uponthe subscriber's level of service. Also, by allocating an overlap ofbands as between first and second oscillators 341 and 342, for example,all eight channels of the midband may be jammed by mean of the presentinterdiction apparatus leaving two channels of the highband which stillmay be jammed via oscillator 342. Consequently, according to FIG. 4,oscillator OSC1 may jam four of the six allocated channel frequencies ofthe midband while oscillator OSC2 may jam an overlapping band comprisingchannels 19-22 of the midband and channels 7-10 of the highband. Therange of jamming signal frequencies for oscillator OSC2 is selectedwithin the range of 150-194 megahertz consistent with the desirableelimination of harmonic interference.

Consistent with these design principles, no band overlap is shown foroscillator OSC3 or oscillator OSC4. Nevertheless, the respectivefrequency ranges of 198-254 megahertz and 258-326 megahertz of theseoscillators eliminate any danger of harmonic interference. Low passfilters 353 and 354 cut off harmonic frequencies above the upper limitsof these respective ranges. Oscillator OSC3 provides jamming signals forjamming four premium channels selected from channels 11-13 of thehighband and channels 23-29 of the superband. Eight premium channels maybe jammed at a reduced jamming factor for these ten channels. OscillatorOSC4 is provided for jamming from channel 30 in the superband to channel41 extending into the hyperband. Four channels of these twelve may bejammed at a 20% jamming interval; however, eight may be jammed at areduced jamming factor.

Microprocessor 300 is connected by a bus system to memory and buffercircuits comprising RAM's 311 and 312 and buffer 310. Microprocessor 300operates at a clock frequency of, for example, four megahertz providedby clock 336. Counter 335 is shown as a separate element; however,counter 335 is provided essentially for counting the output frequenciesof jamming oscillators 341-344 during a calibration mode of operationand so may comprise an element of microprocessor 300.

Microprocessor 300 is also connected to digital to analog converter 320which converts a ten bit voltage control word to analog voltage outputswhich are, in turn, provided to analog multiplexer 330. The analogvoltage outputs of the analog multiplexer 330 are stored and held atsample and hold circuits 337-340 for application to oscillator 341-344.Via a two bit parallel select bus, analog voltage signal outputs aresequentially gated by analog multiplexer 330 over leads FREQ 1-4 towardthe oscillators 341-344. In accordance with the principles of thepresent invention, these signals may be provided in a pseudorandomsequence to thwart pirating attempts as will be described in referenceto FIG. 6.

Microprocessor 300 is connected to each oscillator 341-344 viarespective oscillator power lines OPWR1-4 for actuating the oscillators.Each oscillator may be powered down during a normal mode of operation ifa subscriber is authorized to receive all channels within its allocatedband at one point in time. Furthermore, during a calibration mode, oneoscillator may be powered up for calibration while all other oscillatorsare powered down.

Microprocessor 300 is further connected to four high frequency PIN diodeswitches 361-364. During a normal mode of operation, these switches areselectively opened for a brief interval for, for example, sixteenmicroseconds while an associated oscillator changes or hops from onejamming signal frequency output to another. Nevertheless, assuming fourchannel jamming by a particular oscillator at a jam factor of four, afour thousand hertz frequency hopping rate is easily achievable viathese PIN diode switches.

Also connected to the outputs of each oscillator are associated low passfilters which serve to cut off all harmonics or jamming signal frequencyoutputs. These low pass filters may be connected either to the inputs orto the outputs of switches 361-364 although connection in series betweenits associated oscillator and high frequency switch is shown in FIG. 3.

The jamming signal outputs of all four oscillators are combined atsignal combiner 365. From signal combiner 365, the combined output isdirectionally coupled by coupler 370 to programmable prescaler 375 andto signal attenuator 380.

Programmable prescaler 375 is only powered via lead PREPWR when requiredduring a calibration mode. In accordance with a programmable divide-byfactor, a divided down output frequency is provided to microprocessor300 for counting. When powered down, no output signal results.

During a normal mode of operation, the combined jamming signal outputsof attenuator 380 are combined at directional coupler 385 with thepassed incoming broadband television signal from the common controlcircuit of FIG. 2. As the subscriber is presumed to have paid theirbill, switch 389 and amplifier 387 are assumed to be powered. As aresult of the combining of jamming signals with the broadband spectrum(thus far transmitted in the clear), the subscriber will only receive inthe clear premium or restricted programming which the subscriber isauthorized to receive.

Referring more particularly to FIG. 5, there is shown a block schematicdiagram of one embodiment of a feedback loop useful in describing thecalibration mode of operation. The calibration mode, occupying afraction of a second, assures relatively frequency stable operationduring a normal mode of operation. Furthermore, because of thecalibration mode, there is no requirement for the application of slowconventional frequency locking techniques and a high operation frequencyhopping rate of four thousand hertz may be achieved during the normalmode of operation. The embodiment shows the calibration of oneparticular oscillator OSC. The depicted loop indicates an applicationspecific integrated circuit ASIC connected to subscriber modulemicroprocessor 300. This circuit ASIC may be clocked at twice themicroprocessor rate and comprise the previously discussed voltagecontrol word memory RAM as well as programmable prescaler 375. A wordadjust and select bus 501 is shown which may separately access andadjust all voltage control words in voltage control word memory RAM.When addressed, the voltage control word memory is connected via bus 511to digital to analog converter 320. Digital to analog converter 320 isconnected via sample and hold circuit SH to oscillator OSC to whichpower is applied under microprocessor control via lead OPWR. Viadirectional coupler 370, the jamming signal output of oscillator OSC isfed back toward microprocessor 300. At fixed prescaler 376, the highfrequency output is divided down by a fixed divide-by factor. Thedivided down jamming frequency output is then output to programmableprescaler 375. Programmable prescaler 375 is under control ofmicroprocessor 300. Responsive to premium channel frequency datatransmitted from the headend to microprocessor 260 of FIG. 2,microprocessor 260 in turn generates divide by factor and time betweencount data for transmittal to microprocessor 300 via the serialperipheral interface bus (FIGS. 2 and 3). Microprocessor 300 programsthe divide by factor of programmable prescaler 375 via lead 502 andreceives a countable frequency output of programmable prescaler 375 vialead 503. Microprocessor 300 then counts the output at included counter335.

The provision of application specific integrated circuit ASIC assists inminiaturizing the subscriber module of FIG. 3 and relieves the outboardmemory requirements of microprocessor 300. On the other hand, theprovision of a limited voltage control word memory in circuit ASIC mayrestrict the opportunity of microprocessor 300 to relocate addressableslots to other oscillators when one oscillator is powered down as willbe described in greater detail in reference to FIG. 6. The provision ofa second or fixed prescaler in comparison with the single programmableprescaler shown in FIG. 3 is desirable if the frequency range of thetelevision spectrum to be jammed extends into the hyperband.

Referring now to FIG. 6, there is shown one embodiment of a voltagecontrol word memory having sixty-four memory locations with addresses1-64. At every fourth memory location 1, 5, 9 and so on is located avoltage control word associated with a first oscillator. For theconvenience of establishing a convention for discussion, f10 . . . f1Fwill be assumed to refer to sixteen frequency control words for a firstoscillator OSC1 and are numbered in hexadecimal notation from 0-F. Asindicated above in reference to circuit ASIC memory requirements, thesixteen memory slots may be permanently associated with oscillator OSC1;however, such a design choice limits the freedom of reallocating voltagecontrol words to other oscillators.

Voltage control words are entered into voltage control word memory foreach oscillator in sequence provided the oscillator will be applied forjamming. First, it will be assumed that all four oscillators will beapplied, each for jamming four premium channels. As will be seen, thisis a simplified assumption which assumes a subscriber is authorized toreceive no premium channels and, furthermore, it will be assumed thatall premium channels are to be jammed at the same jam factor four.

In this example, sixteen voltage control words will be entered in memoryfor each oscillator, four of which control words may be the same, eachfour similar control words being related to one premium channelfrequency to be jammed. Thus, four groups of four similar control wordsare entered into sixteen memory locations or time slots 1, 5, 9, 13 . .. 61 for oscillator OSC1. These are indicated as of f10 to f1E. In asimilar manner, sixteen voltage control words are entered into memorylocations 2, 6, 10, 14 . . . 62 for oscillator OSC2. These are indicatedas f20 . . . f2E, Then, sixteen voltage control words are entered intomemory locations 3, 7, 11, 15 . . . 63 for oscillator OSC3, indicated asf30 . . . f3E. Lastly, sixteen voltage control words are entered intomemory locations 4, 8, 12, 16 . . . 64 for oscillator OSC4, indicated asf40 . . . f4E.

The calibration algorithm for loading a first ten bit voltage controlword f10 into a first memory location 1 for a first oscillator OSC 1will now be described in some detail. From the down-loaded frequencydata from microprocessor 300, a first programmable divide-by factor istransmitted via lead 502 to set programmable prescaler 375. All otheroscillators OSC2-4 are powered down via leads OPWR2-4, and oscillatorOSC1 is powered up via lead OPWR1 (shown in FIG. 5 as oscillator OSC andlead OPWR respectively).

From the premium channel frequency data, a first ten bit voltage controlword f10 is stored in memory location 1 representing a first bestestimate of jamming frequency by microprocessor 300 via bus 501. Theword is transmitted to digital to analog converter 320 where it isconverted to an analog voltage. The analog multiplexer (not shown inFIG. 5) selects a lead FREQ1 from the multiplexer to oscillator OSC1.Consequently, the analog voltage output of a digital to analog converteris provided to sample and hold circuit SH or 337 for application tooscillator OSC1. Signal combiner 365 (not shown in FIG. 5 forsimplicity) only passes the jamming signal output from oscillator OSC1to directional coupler 370 because all other oscillators OSC2-4 arepowered down at this time. Via directional coupler 370, the jammingsignal output is provided to fixed prescaler 376. Fixed prescaler 376divides down the output frequency of the oscillator OSC1 to a firstfrequency. According to the divide by factor loaded into programmableprescaler 375, the first frequency output of fixed prescaler 376 isfurther divided down to a frequency which may be counted by counter 335of microprocessor 300. Recognizing that the oscillator output frequencymay be hundreds of megahertz and the clock for microprocessor 300 runsat only four megahertz, the frequency provided via lead 503 should besufficiently divided down to be counted with reasonable accuracy. Sincethe fixed time between counts is known to microprocessor 300 having beendownloaded from microprocessor 260, counter 335 counts the frequencyinput on lead 503. The resulting count is compared with the expectedcount and the microprocessor adjusts the control word accordingly. As aresult, microprocessor 300 repeatedly enters the algorithm until thevoltage control word stored in memory as accurately as possible reflectsthe premium channel frequency to be jammed. Then, this process isrepeated four times for four premium channel frequencies to be jammed bythe oscillator OSC.

During the process of loading the four premium channel frequencies for aparticular oscillator in to the voltage control word memory, there aretwo subordinate schemes by which the four voltage control words for asingle premium channel may be intentionally varied. In a firstsubordinate scheme, via headend 100, four different frequencies may beintentionally selected with reference to one premium channel. Given aresolution of 50 kilohertz provided by the least significant bitpositions of a ten bit voltage control word, the four differentfrequencies may be selected by headend 100 anywhere within the range of100 KHz below to 250 KHz above the premium channel video carrier formost effective premium channel jamming. In a second subordinate schememicroprocessor 300 may be programmed to intentionally vary the enteredvoltage control word to be at or about the expected downloadedfrequency, for example, at fifty kilohertz above or below the expectedfrequency. Consequently, if the headend selects only one frequency for afirst premium channel, for example, at 200 kilohertz above the videocarrier, then voltage control words will be entered into memoryequivalent to video carrier plus 150 kilohertz, 200 kilohertz and 250kilohertz. Both subordinate schemes thwart pirates attempting to notchout the jamming signal frequency which is intentionally varied by theseschemes.

Jamming factor is a term related to the loading of the sixteen voltagecontrol words into voltage control word memory for a particularoscillator. A jamming factor is selected for each premium channel and isglobally transmitted from the headend. If four premium channels are tobe jammed by each of four oscillators OSC1-4 and all are to be jammed atthe same jamming interval, each has a jamming factor of four. If asubscriber subscribes to all four premium channels associated withoscillator OSC1, then oscillator OSC1 may be powered down and no voltagecontrol words entered in memory during calibration for this oscillator.If a subscriber subscribes to two of the four channels, themicroprocessor may allocate the sixteen channel words for the firstoscillator to the two unauthorized premium channel frequencies to bejammed. Consequently, the microprocessor may allocate eight controlwords each to jamming the two unauthorized premium channels thusautomatically increasing the jamming interval or depth of jamming basedon the jamming factor and the given reduced level of premium programauthorization. Jamming factor may be intentionally selected, forexample, at a high level, for example, eight for one especiallysensitive program in relation to two other channels to be jammed by thesame oscillator which may be allocated jam factors of four each, thetotal of all such jam factors being equal to the maximum number ofvoltage control words, in this example, sixteen, associated with theoscillator.

As disclosed above, each oscillator has a number of time slots which areallocated to jamming various frequencies which the particular oscillatoris assigned to jam. Preferably, there may be sixteen time slots percycle with each time slot corresponding to a time duration equal to eachother. According to a novel aspect of the present invention, the dwelltime (the amount of time a particular channel is jammed per cycle) maybe varied. Alternatively, or in addition thereto, the number of timeslots per cycle allocated to a particular channel may be varied.

For example, a particular oscillator may have 16 time slots per cycle,with each time slot having a dwell time of approximately 50 us. Thiswould provide the same amount of jamming to each channel associated withthat oscillator. If for example, that oscillator was responsible forjamming 4 channels, it is possible that it may be desirable to jam oneor more of the channels more than another channel to improve the jamfactor for that channel. This increases the security of the channelsselected thereby increasing their jam factor. For example, pay-per-viewchannels and adult entertainment channels may preferably be providedmore security than some other premium channels.

For purposes of example, the following will illustrate some examples ofhow the dwell time and/or time slots can be varied for an oscillatorthat jams 4 channels identified as A, B, C, D where for this example,channel A is a pay-per-view channel, channel B is an adult entertainmentchannel, and channels C and D are premium channels.

FIG. 8A shows an example of a jamming pattern where each of channels A,B, C and D are jammed equally and in an orderly manner. If it isdesirable to provide the most security to channel A, the next most tochannel B and equal amounts to channels C and D, then, the number oftime slots allocated to a particular channel may be varied as shown inFIG. 8B. This enables greater security for channel A since channel A nowhas a jam factor of approximately 50% (8/16). Channel B has a jam factorof about 25% (4/16) while channels C and D each have a jam factor ofabout 12.5% (2/16). These jam factors ignore the transition timesbetween time slots but are useful approximations for sake of example.

It should be noted that these same jam factors may also be accomplishedby the configurations shown in FIGS. 8C and D. Moreover, a combinationof two or more of these configurations can be used to randomly changethe patterns while maintaining the same jam factors to furthercomplicate the pirating of signals by a would be pirate.

It should be noted that these jam factors can be accomplished by varyingthe number of time slots and concomitantly varying the duration of eachslot. For example, with reference to FIG. 8C, there may only be 8 timeslots with the first time slot having a duration of 4T, a second timeslot with a duration of 2T, a third slot with a duration of T, a fourthslot with a duration of T and a fifth, sixth, seventh and eighth timeslot with durations of 4T, 2T, T and T, respectively. T is preferablybetween 30-100 us.

The amount of time allocated to jamming any channel can be referred toas the dwell time. Therefore, the dwell time can be varied in a varietyof ways as shown in FIGS. 8A-D. Information regarding the desired dwelltime and the order in which channels are to be jammed (jamming channelconfiguration) can be periodically changed by a headend operator. Forexample, it may be desired to jam some channels more at night thanduring the day or more on weekends than on weekdays.

Voltage control words may be read from memory or written into memory sothey may be read out in a particular pseudo random sequence (asexemplified above) so that a pirate would have to know the pseudorandomsequence in order a to appropriately time any notch filtering. Forexample, let f11-f14 be the four premium channel frequencies to bejammed by oscillator OSC1. Addresses 1, 5, 9, and 13 may store voltagecontrol words for f11, f12, f13 and f14, respectively. However, the nextfour addresses 17, 21, 25, and 29 may store the voltage control words ina different order, for example, f14, f13, f12, f11 respectively. Theorder may be further varied in the remaining eight addresses so, whenthe voltage control words are applied to oscillator OSC 1 during anormal mode of operation, the output frequency of the jamming signalwill vary according to the pseudorandom sequence of data entry.

The calibration mode is entered at initial turn-on generate thesixty-four voltage control words storage in voltage control word memorycorresponding to the desired jamming signal frequencies. Periodically,the subscriber module reenters the calibration mode to update thecontrol words for drift which may result from either the oscillator orthe digital to analog converter operation. Such drift if maintainedwithin, for example, 50 kilohertz of the selected frequency is actuallydesirable in that it further complicates the efforts of a would-bepirate. Also, as already indicated the periodically performedcalibration mode permits a higher rate of frequency hopping, forexample, four kilohertz during normal operation than would be possiblewith conventional frequency control methods such as phase locked loops.Calibration requires but a fraction of a second and, consequently,little intelligible television information may be obtained at atelevision receiver tuned to an unauthorized premium channel.

Referring now more particularly to FIGS. 6 and 7 with reference to theschematic block diagram of FIG. 3. the normal mode of operation will nowbe explained. Referring first to FIG. 3, microprocessor 300 uponentering a normal mode of operation causes a first voltage control wordstored in memory address 1 of the voltage control word memory of FIG. 6to be transmitted toward oscillator OSC1. Digital to analog converter320 converts the ten bit word 0010110101 to an analog voltage level.Under control of a bit select bus, analog multiplexer 330 selects leadFREQ1 for transmitting the analog voltage signal for storage and holdingat sample and hold circuit 337. All four oscillators are presumed OSC1provides a jamming signal frequency output FREQ2 consistent with theanalog voltage signal input provided via analogy multiplexer 330.

Referring to FIG. 7, the normal mode of operation, for the example underdiscussion, is shown in the form of a timing diagram. At the output ofthe digital to analog converter is shown, at time t0, an analog voltagelevel representing frequency FREQ 1 for oscillator OSC1. Also, duringtime interval t0-t1 tile analog multiplexer 330 is shown connecting thedigital to analog converter 320 to oscillator OSC1. While the analogmultiplexer is only connected to oscillator OSC1 for the duration t0-t1,the applied analog voltage is stored and held for the duration t0-t4.Consequently, the output of oscillator OSC1 is shown continuouslyapplied from time t0-t4.

Under control of microprocessor 300, via lead OSSW1, switch 361 isbriefly opened while frequency FREQ 1 is established at the output ofoscillator OSC1 and then immediately closed. Switch 361 stays closed forthe duration until the output of oscillator OSC1 hops from frequencyFREQ 1 to FREQ 2. Just prior to time t4, switch 361 is again opened inaccordance with signal OSSW1. Consequently at the output of switch 361,the jamming signal output of oscillator 341 is briefly interrupted.

At time t4, the digital to analog converter 320 is signaled to changethe output frequency of oscillator OSC1 to frequency FREQ 2. As before,the analog multiplexer 330 gates an analog voltage level, this timerepresenting frequency FREQ 2 to be held at sample and hold circuit 337.As a result, oscillator OSC2 now provides a jamming signal frequencyoutput consistent with frequency FREQ 2 until time t8.

Meanwhile, switch 261 which was opened shortly before time t4 inaccordance with switch control signal OSSW1 is again closed at a pointin time shortly after time t4. At any point in time during a normal modeof operation when one of the high frequency switches 361-364 is opened,there will result a loss of a portion of the overall jamming intervalduring which a jamming signal would be applied. Nevertheless, theresulting danger of the presence of no switches 361-364 is that during ahopping from one frequency to the next, an undesirable transition signalmay result at a frequency and level which any distort authorized premiumprogramming. If four premium program channel frequencies are to bejammed by a particular oscillator, each such period of an open state ofa normally closed high frequency switch 361-364 amounts to no more than5% of the overall interval t0-t64 (not shown).

In a similar manner, a first frequency FREQ 1 is established foroscillator OSC2. Referring again to FIG. 6, it will be seen that atmemory address 2 is voltage control word 1010010110 which is providedtoward oscillator OSC2. In accordance with FIG. 7, at time t1 an analogvoltage level is output from digital to analog converter 320representing this word. At a time just prior to time t1, switch 362 isopened in accordance with signal OSSW2. Once frequency FREQ 1 isestablished at the output of oscillator OSC2 or at a time just aftertime t1, switch 362 is again closed in accordance with signal OSSW2provided by microprocessor 300.

As the normal mode of operation continues, all sixty-four memorylocations shown in FIG. 6 are sequentially addressed and provided foroperating oscillators OSC1-4. In accordance with FIG. 7, only the firstseven words are represented as having been provided for selecting thefirst three frequencies for oscillator OSC1 and two frequencies each foroscillators OSC2-4: however, the process for controlling all sixteenfrequencies for each oscillator may follow in the sequence shown orintentionally vary.

In order to thwart pirates and referring to FIG. 7 for oscillator OSC1,it may be seen how frequencies may be output in a pseudorandom sequence.Output frequencies FREQ 1, FREQ 2, FREQ 3, FREQ 4 are shown output inintervals t0-t4, t4-t8, t8-t12, and inferentially, t12-t16 respectively.In the next intervals, the frequencies may be provided, in stead, in thesequence FREQ 4, FREQ 3, FREQ 2, and FREQ 1. Then, in the nextsuccessive intervals the frequencies may be provided in yet a thirddifferent sequence, for example, FREQ 2, FREQ 3, FREQ 4, FREQ 1. Duringthe last four successive intervals extending from t48 to t64, the orderof applied frequencies may be altered again, for example, FREQ 3, FREQ4, FREQ 1, FREQ 2. The pseudorandom sequence may be defined anddownloaded from the headend or developed internally by eithermicroprocessor 260 of FIG. 2 or microprocessor 300 of FIG. 3.

We claim:
 1. An interdiction apparatus used in a cable television system having a headend for preventing unauthorized reception of programming transmitted to a subscriber comprising:jamming means for jamming each of a plurality of unauthorized channels with a controlled jamming signal; and control means for programmably controlling, during a jamming cycle wherein each of said plurality of unauthorized channels is jammed, said jamming means to selectively jam each of said plurality of unauthorized channels for a variable proportion of said jamming cycle, said variable proportion of said jamming cycle being determined by an addressed communication from said headend to said control means.
 2. The apparatus of claim 1 wherein said control means for determining said variable proportion of said jamming cycle comprises:means for assigning a predetermined number of time slots per jamming cycle and means for assigning a variable number of said time slots per jamming cycle to frequencies within each of said plurality of unauthorized channels for which selective jamming is desired, said variable number of said time slots per jamming cycle assigned to said frequencies within each of said plurality of unauthorized channels determined by said addressed communication from said headend to said control means.
 3. The apparatus of claim 2 wherein at least one of said unauthorized channels is assigned a greater number of time slots than another of said unauthorized channels.
 4. The apparatus of claim 2 wherein the assignment of particular time slots to particular unauthorized channels creates a jamming pattern, further wherein said control means is capable of varying said jamming pattern by varying the assignment of one or more of said time slots from one jamming cycle to the next.
 5. The apparatus of claim 2 wherein said time slots are of substantially equal duration and the amount of jamming per unauthorized channel per jamming cycle is based on the number of time slots assigned to respective channels per jamming cycle.
 6. The apparatus of claim 2 wherein one or more of said time slots are not of equal duration with respect to the duration of another of said time slots and the amount of jamming per unauthorized channel per jamming cycle is based on the duration of the time slots assigned to each unauthorized channel per cycle.
 7. The apparatus of claim 5 wherein the duration of each time slot is between about 30-100 microseconds.
 8. The apparatus of claim 5 wherein the duration of each time slot is about 50 microseconds.
 9. The apparatus of claim 6 wherein the duration of each time slot ranges from about 30 microseconds to about 100 microseconds.
 10. The apparatus of claim 2 wherein the assignment of particular time slots to particular channels creates a jamming pattern, further wherein said control means is capable of varying said jamming pattern.
 11. The apparatus of any one of claims 3, 5 or 6 wherein the number of time slots per cycle assigned to a particular unauthorized channel and the duration of one or more of said time slots may vary, and the amount of time a particular unauthorized channel is jammed in a given jamming cycle is base on the number and duration of time slots assigned to said unauthorized channel for said given jamming cycle.
 12. In an interdiction system for selectively jamming a plurality of unauthorized channels in a cable television system comprising a headend and an interdiction apparatus having control means and controlled oscillator means for generating jamming signals which selectively interdict each of said unauthorized channels during a jamming cycle, said jamming cycle comprising a plurality of time slots, a method for varying the amount of time unauthorized channels are jammed within said jamming cycle, said method comprising the steps of:assigning a variable number of said time slots per jamming cycle to each unauthorized channel, said variable number of said time slots per jamming cycle assigned to each unauthorized channel determined by addressed communication from said headend to said control means; and jamming each of said unauthorized channels by interdicting said channels with said jamming signals for a variable proportion of said jamming cycle, said-variable proportion based on the assignment of the time slots per jamming cycle to each of said unauthorized channels.
 13. The method of claim 12 wherein each time slot is of substantially the same duration and the amount of time an unauthorized channel is jammed in a given jamming cycle depends on the number of time slots assigned to said unauthorized channel for said given jamming cycle.
 14. The method of claim 12 wherein said time slots are not all of equal duration and the amount of time an unauthorized channel is jammed in a given jamming cycle is based on the duration of the time slot or slots assigned to said unauthorized channel for said given jamming cycle.
 15. The method of claim 12 wherein the number of time slots assigned to any unauthorized channel and the duration of one or more time slots may vary, and the amount of time a particular unauthorized channel is jammed in a given jamming cycle is based on the number and duration of time slots assigned to said unauthorized channel for said given jamming cycle.
 16. The method of any one of claims 12, 13, 14, or 15 further comprising the step of:generating a varying jamming pattern by varying the number of adjacent time slots assigned to any unauthorized channel in any given, jamming cycle.
 17. The method of any one of claims 12, 13, 14, or 15 further comprising the step of:generating a varying jamming pattern by varying the time slots assigned to any unauthorized channel in any given jamming cycle.
 18. An interdiction apparatus used in a cable television system having a headend for preventing unauthorized reception of programming transmitted to a subscriber comprising:means for interdicting a plurality of channels for which selective jamming is desired with controlled jamming signals; control means, responsive to an addressed communication from the headend, for programmably controlling the generation of said jamming signals; said interdicting means being capable of selectively jamming a plurality of said channels, each for a predetermined amount of time during a jamming cycle, and wherein the amount of time that any channel is jammed within a cycle can be programmably varied; and means for assigning a predetermined number of time slots per jamming cycle and means for assigning a number of said time slots to frequencies within said channels for which selective jamming is desired, wherein at least one of said time slots assigned to a particular channel does not occur consecutively in time with respect to another of said time slots assigned to said particular channel. 